Does this fit over the existing plate? ...

Replaces the existing pedal cleats on the outside of the shoes (the black part in the picture), yes it is compatible with Genius 5, no it is not really compatible with genius 5 with composite sole (Pro Carbon, only one vertical lane for bolts). If your exising plates (metal things inside the shoe) are only two hole then you should take them out of the inside of your shoes and put the plates that come with these in (although you don't have to they will be more secure this way with 4 bolts holding them as they are supposed to). You may or may not have fit issues as spd cleats don't have much lateral (side-to-side) adjustability. If you already have the four bolt metal plates, you just need to buy SPD cleats which should be available separately on their own.